Preprotein translocase is essential for the viability of E. coli and presumably all bacteria The objective of this research proposal is to' relate the role of subunit associations to the catalytic mechanism of preprotein translocase. Two distinct steps of the translocation reaction will be investigated for the involvement of subunit assembly. The first.specific aim will investigate the role of SecD and SecF subunit associations. Specifically, the stoichiometry of the SecDF complex, the exchange of SecD and SecF between SecDF complexes, and the exchange of the SecDF complex with the SecYEG core of translocase will be determined and related to the catalytic mechanism of preprotein translocase. The second specific aim will examine the stoichiometry of SecY to transiting preprotein polypeptide chains. Understanding the dynamics of holoenzyme assembly will be a significant contribution towards understanding the catalytic mechanism of preprotein translocase. This enzyme represents a potential target for anti-bacterial drugs. A complete understanding of the catalytic mechanism of preprotein translocase may aid ill the development of such drugs.